1. Field of the Invention
The present invention relates to a synchronization method and apparatus, and in particular to a synchronization method and apparatus in a data transmission using a transparent transmission line.
2. Description of the Related Art
In a data transmission, a method for transmitting a synchronizing signal carried on a specific frequency has been known as a synchronizing method of establishing a timing synchronization or a frame synchronization. Also, such a synchronizing method is used in transmission apparatuses such as modems.
First of all, prior art for establishing the timing synchronization and the frame synchronization will be described hereinafter.
(1) Timing Synchronization
The prior art of the timing synchronization includes a QAM (Quadrature Amplitude Modulation) transmission system. In the QAM transmission system, since a data signal is transmitted at a Nyquist interval on a transmission side, it is made possible to establish the timing synchronization on a reception side by extracting Nyquist frequency components by a band-pass filter.
FIG. 17A shows an arrangement of a prior art modem using the QAM transmission system. FIG. 17B shows an example of a transmission signal of this modem.
An operation where this modem is used in, for example, a power-line carrier communication will now be described.
On the transmission side of a modem 10 shown in FIG. 17A, a scramble process is performed to a transmission signal SD by a scrambler (SCR) 11, and a serial signal is converted into a parallel signal. The parallel signal is converted from a Gray code (G) of which the transmission signal is originally formed into a Natural code (N) by a vector sum circuit 12. After a vector sum calculation is performed corresponding to a vector difference circuit 28 for detecting a phase on the reception side, a signal generator 13 transmits the transmission signal.
This transmission signal has its waveform shaped by a roll-off filter (ROF) 14. The output signal of the roll-off filter 14 is modulated by a modulation circuit (MOD) 15 and is further converted from the digital signal into the analog signal by a D/A conversion circuit 16. Then, a low-pass filter (LPF) 17 extracts a signal only in a low frequency band including a frequency band (10 kHz-450 kHz) of a power carrier wave to be transmitted to the line.
When such a transmission signal is received through the reception line, only predetermined frequency band components (10-450 kHz for a power carrier modem) are firstly extracted by a band-pass filter (BPF) 19, and restored to the digital signal by an A/D conversion circuit 20.
The analog signal expressed in the digital form is demodulated into the baseband signal by a demodulation circuit (DEM) 21, and then the waveform is shaped by a roll-off filter 22.
The output of the roll-off filter 22 is sent to a synchronizing (sync) signal extractor 23 and a VCXO type PLL circuit 24, whereby a phase of a synchronizing signal (timing synchronizing signal) is extracted and a sampling timing signal is provided to the A/D converter 20.
An intersymbol interference is removed from the output signal of the roll-off filter 22 by an equalizer (EQL) 25, and a phase adjustment is performed by a carrier automatic phase controller (CAPC) 26, so that a decision circuit (DEC) 27 further outputs only signal components.
A vector difference (error) calculation by the Natural code, opposite to the vector sum circuit 12, is performed by the vector difference circuit 28, and the Natural code is restored to the Gray code, so that the parallel Gray code is converted into a serial signal by a descrambler (DSCR) 29 for the descramble process to be outputted as a reception signal RD.
In addition, a transmission clock generation circuit (TX-CLK) 18 provides a transmission clock to the D/A converter 16, and distributes the same to other portions. Also, on the reception side, a reception clock generation circuit (RX-CLK) 30 extracts the reception clock to be provided to the portions of the receiver.
Also, the hatched portion of FIG. 17A corresponds to a Nyquist transmission line 31 that is transparent. FIG. 17B shows an example of a transmission signal in case the signal generator 13 generates a transmission signal at 192 kB. In this case, the Nyquist interval of the Nyquist transmission line 31 assumes 192 kB.
FIG. 18 specifically shows the synchronizing signal extractor 23 and the VCXO type PLL circuit 24 shown in FIG. 17A.
The synchronizing signal extractor 23 is composed of a power calculator (PWR) 231, a band-pass filter 232, and a vectorizing circuit 233. The PLL circuit 24 is composed of a comparator 241, a low-pass filter 242, a secondary PLL circuit 243, a D/A conversion circuit 244, a VCXO (Voltage Controlled Crystal Oscillator) circuit 245, and a frequency divider 246.
Namely, the vector signal outputted from the roll-off filter 22 is squared by the power calculator 231 to calculate the power. The power value thus obtained is passed through the band-pass filter 232. Since the band-pass filter having the Nyquist frequency 192 kHz as the center frequency is used in this example, the synchronizing signal of the Nyquist frequency is outputted to the vectorizing circuit 233.
“Vectorization” will now be described referring to FIGS. 19A and 19B. In FIG. 19A, while a solid line shows a waveform of the input signal (192 kHz) for the vecrorizing circuit 233, and a dashed line shows a waveform where a phase of this input signal is shifted by 90 degrees.
In FIG. 19A, supposing the amplitude is “1”, values of the input signal and the signal whose phase is shifted therefrom by 90 degrees at each of points A, B, C, and D are (1, 0), (0, −1), (−1, 0), and (0, 1), respectively. Being plotted on xy coordinates, they rotate along a circle shown in FIG. 19B in the order of A→B→C→D. Therefore, it is seen that the input signal of scalar value is vectorized by the signal whose phase is shifted therefrom by 90 degrees.
Describing FIGS. 19A and 19B together with FIG. 18, the output of the vectorizing circuit 233 is provided to the PLL circuit 24 as timing phase information.
In the PLL circuit 24, the timing phase information from the vectorizing circuit 233 is firstly compared with the phase of a reference point preknown at the comparator 241. When there is a phase difference due to a transmission line delay, a point which should be located inherently at e.g. the point A in FIG. 19B is shifted to a position of a point X. In this case, a phase difference θ between the points A and X is detected by the comparator 241 and passed through the low-pass filter 242.
Since the low-pass filter 242 extracts only the low band component, high frequency variations in the phase difference θ caused by the noise are removed. With the phase difference θ thus obtained, the controlled voltage of the VCXO 245 is controlled by the secondary PLL circuit 243 composed of two integrators and the D/A conversion circuit 244. After performing the frequency division at the frequency divider 246, the phase information is fed back to the comparator 241 to be compared with the phase at the reference point.
Thus, the phase difference θ between the timing phase information from the vectorizing circuit 233 and the reference point is pulled in or nullified, thereby enabling the extraction of the synchronizing signal whose synchronization is established. Also, the sample timing signal to the A/D converter 16 is outputted from the VCXO circuit 245, and is finally fed back to the comparator 241 to form a phase locked loop.
(2)Frame Synchronization
The prior art of the frame synchronization includes a DMT (Discrete MultiTone) system, or an OFDM (Orthogonal Frequency Division Multiplexing) system. In the DMT system/OFDM system, a specific frequency is allocated to a pilot signal, and the frame synchronization is established on the reception side by extracting the pilot signal.
FIG. 20 shows a modem using the DMT system/OFDM system. The difference between the arrangements of a modem 90 shown in FIG. 20 and the modem 80 shown in FIG. 17A is that the modem 90 is provided with a DMT multiplexer 61 instead of the role-off filter 14 and the modulation circuit 15 on the transmission side of the modem 80. Also, the modem 90 is provided with a DMT distributor 62 instead of the demodulation circuit 21 and the role-off filter 22 on the reception side of the modem 80.
It is to be noted that the DMT multiplexer 61 composes a frame (master frame) by multiplexing the transmission signals (DMT signals for a predetermined channel number) transmitted from the signal generator 13 with an inverse fast Fourier transform (IFFT), thereby inserting a pilot signal into a specific frequency. Also, the DMT distributor 62 demultiplexes the reception signal with a fast Fourier transform (FFT) to extract the frame.
As for the synchronizing signal extractor 23 and the VCXO type PLL circuit 24 in FIG. 20, those having the same arrangement as that in FIG. 18 may be used. However, the band-pass filter 232 has a frequency of the pilot signal as its center frequency. Also, the power calculator 231 is made unnecessary in this case.
Systems such as the QAM system and the DMT system/OFDM system where the synchronizing signal is carried on a specific frequency for the transmission are effective when the transmission of the specific frequency is guaranteed. However, supposing a power-line carrier communication, for example, since a power line cannot guarantee the transmission of the specific frequency greatly influenced by noises, the establishment of synchronization has been difficult.
This is because the power line exhibits an extremely complicated line characteristic. This will be described referring to FIGS. 21 and 22A-22C.
In a power system shown in FIG. 21, power in a distribution substation 100 is firstly supplied to a pole transformer 103 through a 6.6 KV high voltage distribution line 102, and is further supplied to a home 105 through a 100 V/200 V low voltage distribution line 104.
Upon performing a power-line carrier communication, an optical fiber (not shown) is set up parallel with the high voltage distribution line 102 between an access node 101 in the distribution substation 100 and a modem set up in the pole transformer. Through the optical fiber, the communication between the modem in the pole transformer 103 and the modem inserted into the convenience outlet connected to an interior distribution line 106 in the home 105 is performed through the 100 V/200 V low voltage distribution line 104.
In this case, as shown in FIGS. 22A-22C, the low voltage distribution line 104 appears to be an inductor of 1 μH/m, as shown in FIG. 22B, for a spectrum of a transmission signal TX shown in FIG. 22A, and appears to be an inductor of 150 μH if the line length is assumed to be 150 m.
Also, a service wire 107 connected to the low voltage distribution line 104 appears to be a capacitor of 75 pF/m, and appears to be a capacitor of 0.1125 μF if a 50 m service wire is assumed to be connected to the home 105. Not only the service wire, but also various household electric appliances in the home 105 appear to be a capacitive load (see FIG. 22B), since the capacitors for canceling the noise are connected to AC 100 V.
Consequently, the portion between the utility pole where the pole transformer 103 is placed and the convenience outlets in the home appears to be a low-pass filter (LPF), as shown in FIG. 22B, and a reception signal RX greatly attenuates in a high frequency band, as shown in FIG. 22C. Therefore, when arriving at the terminal side, the high frequency band signal is buried in a noise N in the worst case.
On the other hand, although the loss in the low frequency band is not so large compared with the high frequency band, noises from the household electric appliances such as inverter appliances are extremely high. Therefore, the received signal is also buried in the noise N as shown in FIG. 22C.
Supposing the Nyquist frequencies of the above-mentioned QAM transmission system are frequencies f1 and f3, and the pilot frequency of the DMT system/OFDM system is a frequency f2, the transmission of these frequencies in the power-line carrier communication is not guaranteed as shown in FIG. 22C. Therefore, establishment of synchronization using such a specific frequency has been difficult.